The present invention relates to a process for preparing colored particles. More specifically, the present invention is directed to a process which comprises preparing polymeric particles and subsequently reacting the polymeric particles with a dye to form a polymeric particle having a dye covalently bonded thereto. Particles prepared according to the process of the present invention are useful, for example, as colorants for liquid electrophotographic developers; thus, the present invention is also directed to liquid developer compositions comprising a liquid medium, a charge control agent, and the colored polymeric particles prepared according to the process disclosed herein.
Many methods are known for the preparation of polymer particles. For example, dispersion polymerization has been found to be a convenient method of preparing particles in a fluid medium; thus, polymeric particles having a diameter of up to 4 microns may be prepared in an aliphatic medium according to this method, as disclosed, for example, in "Luminescence Techniques in Polymer Colloids. 1. Energy-Transfer Studies in Non-Aqueous Dispersions", O. Pekcan, M. A. Winnik, L. S. Egan, and M. D. Croucher, Macromolecules, Vol. 16, pages 699-702 (1983), the disclosure of which is totally incorporated herein by reference. Processes for coloration of polymeric particles are also known, and include methods such as the use of polymerizable dyes as comonomers during the free-radical dispersion polymerization process in which the polymeric particle is formed, as disclosed in copending application U.S. Ser. No. 783,729/85, the disclosure of which is totally incorporated herein by reference. The process of this copending application comprises dissolving in a suitable solvent a mixture of at least two first monomer components including at least one second monomer having covalently attached thereto a dye moiety, subsequently adding to the resulting reaction mixture a polymerization initiator, thereafter effecting polymerization of the reaction mixture by heating, and separating the product particles therefrom. This process results in a polymeric particle wherein the dye entity is attached to the monomers prior to the polymerization reaction.
Methods of preparing dye labelled polymeric particles are also known. For example, the article entitled "Synthesis and Characterization of Fluorescently Labelled Poly(vinyl Acetate) Particles", L. Egan et al., Journal of Polymer Science, Vol. 24, pages 1895-1913 (1986), discloses a process for preparing nonaqueous dispersions of poly(vinyl acetate) particles stabilized with poly(2-ethylhexyl methacrylate) and labelled with fluorescent dyes in the core and stabilizer polymer phases. For preparation of particles containing a fluorescent dye label in the stabilizer portion, the processes disclosed entails providing a labelled stabilizer and reacting it with the core monomers to form the stabilized polymeric particles, as indicated on pages 1896 and 1903.
In addition, the coloration of polymeric particles by dye imbibition, as disclosed in U.S. Pat. No. 4,476,210, the disclosure of which is totally incorporated herein by reference, is known. This patent relates to a process for making a dyed stabilized liquid developer comprising marking particles dispersed in an aliphatic dispersion medium. The marking particles comprise a thermoplastic resin core with an amphipathic block or graft copolymer steric stabilizer irreversibly chemically or physically anchored to the resin core, and having a dye imbided in the resin core, said dye being soluble in the core material and insoluble in the liquid dispersion medium of the developer. In the developer disclosed in this patent, the dye is dispersed or dissolved in the polymeric particles that constitute the marking particles and no covalently bonded to the polymeric particles.
Another method of preparing colored particles is disclosed in U.S. Pat. No. 4,636,452. This patent discloses a method for producing electrophotographic liquid developers which comprises dissolving a first polymer in a highly insulating hydrocarbon medium that also contains a monomer. The polymer and monomer are selected such that each has a polar functional group having adsorptivity to the other molecule in the solution, and the monomer is then polymerized to produce a second polymer exhibiting low solubility in the medium and existing in substantially particulate form. Polymeric particles prepared according to this method are colored by dispersing or dissolving a dye molecule therein.
Also, a process for preparing dry toner compositions by dispersion polymerization is disclosed in U.S. Pat. No. 4,617,249. The process entails preparing a solvent medium containing dispersed steric stabilizers, monomers, and initiator compounds, adding a crosslinking compound, heating the mixture to polymerize the monomers, and separating the formed particles from the mixture. Coloration of the particles may be effected by diffusing a dye, present in a solvent solution, into the polymer mixture.
British Patent No. 1,181,287 discloses a method for preparing colored polymeric materials useful in electrophotographic developers, which method comprises preparing a material having the colorant covalently bound to it by polymerization of a monomer with a diazotized color material. Free radical dissociation of the diazotized coloring material then occurs, which results in the coloring material coupling to a polymer. Before it is suitable for use in a developer, the colored material is processed by methods such as wet grinding or ball milling. The process of the present invention, unlike the process disclosed in this patent, entails the direct preparation of particles of the desired size, and no processing by grinding or milling is required to produce particles useful for applications such as liquid developers.
Japanese Patent Application No. 54029/78 discloses a liquid toner for electronic photography comprising a mixture of a first liquid toner and a second liquid toner, at least one of which contains a dielectric agent. The first liquid toner contains a nonpolar solvent, a thermoplastic synthetic binder dissolved in the solvent, and a dispersant. The second liquid toner contains a nonpolar solvent capable of being mixed with the first liquid toner; a graft amphipathic polymer having a polymer main chain portion and a polymer graft chain portion grafted to the main chain portion to form the amphipathic polymer, wherein one portion can be dissolved in the solvent and acts as a binder and a dispersant, and the other portion cannot be dissolved in the solvent but functions as a binder, and wherein at least one of the two components is thermoplastic. Regarding the second toner, the polymer component soluble in the solvent is dissolved in the solvent forming a continuous phase, while the polymer component insoluble in the solvent forms a dispersed phase resulting in the amphipathic polymer acquiring the functions of a binder and a dispersant as a single dispersed phase. The second toner may contain a colorant, which colorant may be either a pigment or a dye, and which colorant may be attached to the polymeric particles colloidally suspended in the liquid either chemically or by secondary forces or surface adhesive forces. If a dye is used as a chemically bonded colorant, the dye can be bonded with the amphipathic polymer after it is formed, or it can be bonded with the precursor or grafted side chain portions of the polymer prior to its formation, as indicated on pages 46 to 49 of the translated text. When the coloration process occurs by means of a chemical reaction between a dye and the amphipatic polymer, the reaction takes place between the dye and the reactive groups in the dispersed or insoluble phase, and not between the dye and reactive groups in the soluble phase. The patent does not, however, disclose the coloration of polymeric particles by reacting a dye molecule with reactive groups attached to the soluble stabilizing polymers.
Additionally, U.S. Pat. No. 3,753,760 discloses a liquid toner with a number of solids less than those conventionally used in a multicomponent liquid toner, obtained by combining the functional characteristics of plural previous different kinds of solids into a complex molecule, thereby obtaining better image fixation, improved resistance to preferential depletion, improved image definition, clear background, improved shelf life, improved functional life, and a broad color range. The toner comprises a liquid solvent system, an amphipathic polymeric molecule of the graft type having a polymeric backbone part and a polymeric graft part on said backbone part, said molecule being comprised of two moieties, at least one of which is thermoplastic, wherein one moiety is solvated by the solvent system and is a fixative and a dispersant, and the other moiety is insoluble in the solvent system, has a particle size between 25 millimicrons and 25 microns, and acts as a fixative, such that the polymeric molecule acts a mono-dispersed particle phase, a fixative, a dispersant, and a charge director. A coloring agent may be included as a moiety of the amphipathic molecule by means of a chemical bond, which bonding can be either to the precursor portions of the molecule prior to the graft or block polymerization of the added chains or to the chains added by graft or block polymerization. When the coloration process occurs by means of a chemical reaction between a dye and the amphipathic polymer, a reaction takes place between the dye and reactive groups in the dispersed or insoluble phase, and not between the dye and reactive groups in the soluble phase. The patent does not, however, disclose the coloration of polymeric particles by reacting a dye molecule with reactive groups attached to the soluble stabilizing polymers.
Although the prior art processes and compositions are suitable for their intended purposes, a need continues to exist for processes for preparing colored polymeric particles wherein the colorant is covalently bonded to the particle. A need also exists for a process wherein colored polymeric particles of a desired size may be prepared directly with no need for further processing to obtain particles of the desired size. In addition, a need exists for a process for preparing non-toxic colored polymeric particles. Further, there is a need for a process for obtaining dyed polymeric particles wherein the dye will not leach out from the particles in a liquid medium. Also, a need exists for a process for preparing colored particles suitable for use in electrophotographic liquid developers. A need also exists for a process for preparing colored particles wherein a wide choice of colors for the particles is available. In addition, there is a need for a process for preparing colored polymeric particles wherein the dye is covalently bonded to the polymer subsequent to the particle synthesis process, rather than prior to the preparation of the particles, since (1) dyes may undergo color changes when subjected to the conditions of a polymerization reaction, (2) the presence of dye molecules on the polymer prior to preparation of the particles can alter the solubility characteristics of this polymer and its suitability as a stabilizer, and (3) the presence of dye molecules on the stabilizer can affect the size and size distribution of the particles as well as the amount of stabilizer, and as a consequence the amount of dye incorporated into the particle. In addition, there is a need for a process for the coloration of particles in which the dye molecule is covalently bonded to the stabilizing polymer portion of the particles rather than to the polymeric cores of the particles. Also, a need exists for a process for the preparation of colored polymeric particles wherein two or more different dyes are covalently bonded to the polymer subsequent to the particle synthesis. There is also a need for a process for the coloration of particles in which the dye molecules are dispersed in the polymer in a manner that prevents dimerization or aggregation of the dye molecules, since dimers and aggregates often have a color different from that of the isolated dyed molecules, and particles containing these aggregates may possess reduced transparency. In addition, there is a need for liquid electrophotographic developer compositions containing colored particles having a dye covalently obtained to a stabilizing copolymer portion of the particle. A need exists for liquid developer compositions that are non-toxic and for which a wide range of color choices is available. Further, a need exists for a method of imaging with liquid developer compositions containing the above mentioned colored polymeric particles.